1. Field of the Invention
The present invention relates to a developing apparatus used in an image forming apparatus with an electrophotographic system or an electrostatic recording system, such as a copying machine, which visualizes an electrostatic image formed on an image bearing member by fixing developer thereto and an image forming apparatus having the developing apparatus.
2. Description of the Related Art
Recently, for an image forming apparatus with an electrophotographic system, such as a copying machine and a printer, there has been a strong demand for downsizing the apparatus main body in order to save space. In particular, for an image forming apparatus of a full-color mode, the demand for downsizing is growing since a plurality of developing apparatuses are mounted. An invention of a developing apparatus which fulfills such a demand has been disclosed in Japanese Patent Application Laid-open (JP-A) No. 5-333691.
The developing apparatus disclosed in JP-A No. 5-333691 includes a developing chamber for supplying developer to a developing sleeve and an agitating chamber for collecting the developer after developing into a developer container. The developing chamber and the agitating chamber are arranged up and down. Such arrangement is described with reference to FIG. 2 in this application.
The developing apparatus 1 in FIG. 2 has a feature that two conveying screws 5, 6 as developer circulation unit are arranged up and down. The developing apparatus 1 has the developer container 2 for accommodating the developer. An opening portion 2a is formed at a part of the developer container 2 opposed to a photosensitive drum 10. A developing sleeve 8 as a developer bearing member is provided at the opening portion 2a. 
Further, a partition wall 7 extending toward the opening portion 2a is formed at an inner wall of the developer container 2. The partition wall 7 partitions the developing chamber 3 and the agitating chamber 4. The developing chamber 3 is arranged at the upper side in the developer container 2 and the agitating chamber 4 is arranged at the lower side in the developer container 2. A first conveying screw 5 and a second conveying screw 6 are provided respectively in the developing chamber 3 and the agitating chamber 4 as the circulation unit for circulating the developer within the developer container 2 while agitating and conveying the developer. The first conveying screw 5 conveys the developer in the developing chamber 3 to the developing sleeve 8. The second conveying screw 6 uniforms toner density of the developer by agitating and conveying toner which is supplied to the upstream side of the second conveying screw 6 in the agitating chamber 4 from a toner supplying port (not illustrated) with the developer which is previously contained in the agitating chamber 4.
With the developing apparatus 1 of such a vertical agitation type, the occupying space in the horizontal direction is saved since the developing chamber 3 and the agitating chamber 4 are vertically lined. Accordingly, for example, downsizing in the horizontal direction can be achieved even with a color image forming apparatus of a tandem type having a plurality of developing apparatuses 1 mounted in parallel in the horizontal direction.
In the developing apparatus 1 of the vertical agitation type, the developer is drawn to the developing sleeve 8 with a magnetic pole N1 of a magnet roller 8a as magnetic field generating unit which is provided non-rotatably at the inside of the developing sleeve 8, as indicated by arrow a in FIG. 2. In this case, the magnetic pole N1 is arranged within the developer container 2.
When the developing sleeve 8 is rotated, the developer is conveyed from the inside to the outside of the developer container 2 by being sequentially conveyed to a magnetic pole S1 and then to a magnetic pole N2 which is at a part of the developer sleeve 8 exposed to the photosensitive drum 10 side. Then, the developer arrives at a developing area having a magnetic pole S2 which is opposed to the developing sleeve 8 and the photoconductive drum 10. At the midway of conveying the developer, the thickness of the developer is magnetically regulated in cooperation with a developer regulating edge 9 as a developer regulating member and the magnetic pole S1 which is arranged to be opposed thereto. Thus, the layer of the developer is thinned and an electrostatic image is developed at the developing area.
The remaining developer without being used for the developing at the developing area is conveyed into the developer container 2 with a magnetic pole N3 which is arranged at the downstream side of the developing area in the rotation direction of the developing sleeve 8. The developer is removed from the developing sleeve 8 by a repulsing magnetic field of the same polarity magnetic poles N1, N3 which are adjacently arranged at the inner side of the developer container 2. Then, the developer is collected into the agitating chamber 4 which is defined at the lower part in the developer container 2.
In this case, the developer is not collected into the developing chamber in accordance with the rotation of the developing sleeve 8 unlike a developing apparatus of the horizontal agitation type. As indicated by arrow c in FIG. 2, the developer is collected into the agitating chamber 4 which is defined vertically below the developing chamber 3. Thus, the only developer which is sufficiently agitated in the agitating chamber 4 invariably exists in the developer container 2. In this manner, the developing sleeve 8 is invariably provided with the developer of uniformed density so that steady images without unevenness and darkness difference in the thrust direction can be obtained.
Meanwhile, the image forming apparatus is required to be capable of accepting various media. For example, the process speed can be set variously and is set slow in the case of a thick sheet. At that time, when the developer conveyance capacity of the second conveying screw 6 is set smaller than that of the developing sleeve 8, the developer is accumulated at the upstream side of the second conveying screw 6 and the circulation of the developer becomes out of balance. For this reason, the developing sleeve 8 and the second conveying screw 6 are independently driven and set to be at the appropriate rotation speed for each process speed so as to prevent the developer circulation from being out of balance at any process speed.
However, with the configuration of the developing apparatus 1 of the vertical agitation type in JP-A No. 5-333691, the following problem remains with the developer conveyance.
There is a difference between inertia forces of the developing sleeve 8 and the second conveying screw 6. Accordingly, there arises a difference between a stop turnaround time until the rotation of the developing sleeve 8 is stopped based on a drive stop trigger signal and a stop turnaround time until the rotation of the second conveying screw 6 is stopped based on a drive stop trigger signal.
Here, it is assumed that a motor 11 of the developing sleeve 8 and a motor 12 of the second conveying screw 6 simultaneously receives the drive stop trigger signal. In this case, due to the difference of the stop turnaround times, the developing sleeve 8 is rotated longer than the second conveying screw 6, for example. Accordingly, there may be a case that the developer is excessively conveyed from the developing chamber 3 to the agitating chamber 4 via the developing sleeve 8. In the case that redundant rotation of the developing sleeve 8 is repeated corresponding to repeating of the drive start and the drive stop of the developing sleeve 8 and the second conveying screw 6, the developer circulation becomes out of balance. Then, there is a risk that, in the end, the developer overflows from the developing apparatus 1.
The present invention provides a developing apparatus which is capable of suppressing overflowing of developer from the developing apparatus due to imbalance of developer circulation even with difference between stop turnaround times caused by difference between inertia forces of the developer bearing member and a conveying member.